People have harnessed the sun's energy since ancient times. By circa 400 B.C.E., the Greeks knew that glass spheres filled with water could concentrate the sun's rays and start fires. By circa 200 B.C.E. the Greeks and Chinese used curved mirrors to light fires with sunlight.
From 900 C.E. to about 1300 C.E. the Anasazi Indians of what is now the southwestern U.S., built dwellings with massive south facing stone or adobe walls. These walls absorbed heat during the day and released it at night.
Modern, flat plate collectors evolved from early experiments with hot boxes. Horace de Benedict de Saussure, a Swiss scientist, built the first hot box in 1767. He used a glass covered wooden box with cork insulation to collect sunlight. William J. Bailey, an American engineer, developed the first flat plate collector in California in 1909.
In 1940, George Fred Keck, an American architect, built the first modern passive solar home in a Chicago suburb. Most of the south wall was covered by windows made of two panes of glass with a thin layer of air sealed between them. Engineers at Bell Telephone laboratories created on efficient photovoltaic cell in 1954.
During the 1970's and 1980's shortages of oil and natural gas led to the development of more efficient solar technology. In some areas, new solar homes have become an economical alternative to homes heated with gas or electricity. Experts believe that, as people use direct solar energy more and more, the mass production of solar equipment and the development of better equipment, will steadily improve the economic advantages of using direct solar energy.